Large pulverized coal boilers, which may be used in power stations or in factories, are equipped with a smoke duct fitted with heat exchanger tubes, with the smoke and the accompanying fine particles of ash passing round the outside of the heat exchanger tubes.
Such heat exchangers may be horizontal or vertical, and they are constituted by parallel grids comprising a main region having rectilinear tubes extending perpendicularly across the direction of smoke movement, and two end regions of smaller size having bends and lengths of tube lying parallel to the direction of smoke movement.
The average speed of smoke movement through such heat exchangers is generally limited to between 10 and 15 meters per second (m/s) in order to avoid the tubes being eroded by the coal ash, which may be highly abrasive, depending on the nature of the coal.
However, by virtue of their particular shape and the inevitable gaps between the walls of the duct and the closest adjacent tubes lying parallel to the direction of smoke movement, the end regions have a lower head loss coefficient than the main region. As a result, large local excess speeds generally occur in the end regions, which speeds may, for example, be as much as twice the average design speed of smoke movement, thereby causing the tubes to be locally eroded. This leads to major re-conditioning operations which are expensive and which take the boiler out of service.
Prior art devices are generally constituted by obstacles disposed upstream from the heat exchangers in order to provide a baffle effect. Such devices only protect the inlet to the heat exchangers and therefore do not provide them with complete protection. Further, regions of major excess speed may form at the ends of such obstacles. and may be accompanied by considerable concentrations of ash, thereby giving rise to local erosion.
The aim of the invention is to provide complete protection to the heat exchangers by avoiding any local regions of excess speed and any regions of ash concentration.